go_local_sdk: more strict SDK platform detection (#2765)

We determine the "SDK platform" by looking at the goos_goarch
subdirectory of pkg/tool. If the goroot has actually built the toolchain
for multiple platforms, then we just pick the first one and potentially
fail cryptically later.

Ideally, we would simply register toolchains for all of the host
platforms we find, but that is likely a much larger change. Until then,
provide a much clearer error message that we don't like finding multiple
platforms.

Fixes #2764

go/private/sdk.bzl

@@ -252,13 +252,21 @@ def _detect_host_sdk(ctx):
     return root
 
 def _detect_sdk_platform(ctx, goroot):
-    res = ctx.execute(["ls", goroot + "/pkg/tool"])
+    path = goroot + "/pkg/tool"
+    res = ctx.execute(["ls", path])
     if res.return_code != 0:
-        fail("Could not detect SDK platform")
+        fail("Could not detect SDK platform: unable to list %s: %s" % (path, res.stderr))
+
+    platforms = []
     for f in res.stdout.strip().split("\n"):
         if f.find("_") >= 0:
-            return f
-    fail("Could not detect SDK platform")
+            platforms.append(f)
+
+    if len(platforms) == 0:
+        fail("Could not detect SDK platform: found no platforms in %s" % path)
+    if len(platforms) > 1:
+        fail("Could not detect SDK platform: found multiple platforms %s in %s" % (platforms, path))
+    return platforms[0]
 
 def _parse_versions_json(data):
     """Parses version metadata returned by golang.org.